This is a test invented by Bernard Williams in "Williams (Bernard) - The Self and the Future" as a means of teasing out whether or not we really think we will be some future individual. It is cashed out in terms of the prospect of “being mercilessly tortured in the morning”.

If we think that unfortunate individual will be us, our attitude will be qualitatively different to if we think it will be someone else (though maybe there are exceptions – mothers and their children, maybe – but even there, it may depend on what is to happen – reference Winston Smith and the rats in Room 101 in 1984 - Wikipedia: Room 101).